Production of acrolein, acrylic acid and water-absorbent polymer structures made from glycerine

ABSTRACT

The present invention relates to a process for the production of acrolein, comprising the following steps:
     (a) bringing into contact of an aqueous glycerine phase in an acrolein reaction area to obtain an aqueous acrolein reaction phase;   (b) depleting the acrolein from the acrolein reaction phase to obtain an acrolein phase and a depleted acrolein reaction phase;   (c) conducting back at least a part of the depleted acrolein reaction phase into the acrolein reaction area.
 
The invention further relates to a process for production of acrylic acid as well as of water-absorbing polymer structures, composites, in particular hygiene articles, comprising these water-absorbing polymer structures, a process for production of the composites and further chemical products based on the acrylic acid obtained by the inventive process and also the use of this acrylic acid in chemical products.

This application is a national stage application under 35 U.S.C. 371 ofinternational application No. PCT/EP2006/005793 filed 16 Jun. 2006, andclaims priority to German Application No. DE 10 2005 028 624.0 filed 20Jun. 2005, the disclosures of which are expressly incorporated herein byreference.

BACKGROUND

The invention relates to a process for production of acrolein, acrylicacid and of water-absorbing polymer structures, and composites, inparticular hygiene articles, comprising these water-absorbing polymerstructures, a process for production of these composites, as well asfurther chemical products based on the acrylic acid obtained by theprocess according to the invention and also the use of this acrylic acidin chemical products.

In GB 141 057 a process for dehydration of glycerine to form acrolein isdescribed, in which the reaction is carried out at about 200° C. at amixture of potassium hydrogensulfate and potassium sulphate. Thisprocess leads, however, to only unsatisfactory selectivities, which, inaddition, significantly decrease in the course of a longer reaction.Thus, this process is poorly suited to industrial use. By selectivity isunderstood the quotient of the molar amount of generated product and themolar amount of a reference component, here, glycerine. For continuouslyoperated systems, the quotient of the molar flow is considered.

Furthermore, FR 695 931 describes another method for dehydration ofglycerine to acrolein at a solid state catalyst. From the repeat of thisprocess carried out in DE 42 38 493, it may be seen that the yields ofthis process are not sufficient for technical use.

In DE 42 38 493, both gas phase and liquid phase reactions at a solidstate catalyst for conversion of glycerine to acrolein are described.With high selectivities, only comparably low turnovers were achieved,which, in addition, decrease with increasing turnover.

Although this process is interesting for an industrial use in view ofthe high selectivities, the turnovers achieved and the reduction ofselectivity are in need of improvement.

In WO 03/051809, a process for production of acrylic acid starting frompropylene via acrolein is disclosed, which is perfectly suited forindustrial production of acrylic acid. Besides propylene, which isgenerally obtained from petrochemical processes, such as naphthacracking, there exits, however, a further route to the production ofacrylic acid, which is not based on a petrochemical but on native(renewable) raw materials, via glycerine, which is produced, forexample, by fat saponification, fat splitting, as well as duringbiodiesel production.

The object of the present invention is first, generally to alleviate oreven to overcome the disadvantages arising from the state of the art.

A further object of the present invention is to provide a process forproduction of acrolein from glycerine, which is suitable for industrialuse and, in particular, has satisfactory turnover and selectivities.

A further object according to the invention is to provide a process forproduction of acrolein, which generates an acrolein phase, which issuitable for feeding into the further step, namely the conversion ofacrolein to acrylic acid by oxidation.

In addition, an object according to the invention is to provide aprocess for production of acrylic acid which may find industrialapplication. Furthermore, polyacrylates, in particular water-absorbingpolyacrylates, also called superabsorbers, are used in manyapplications, so that it is a general requirement to produce thesepolyacrylates at least partially on the basis of renewable raw materialsand thus to provide polyacrylates based at least partially on renewableraw materials. This is of particular interest in particular forwater-absorbing polymers, since the water-absorbing polymers produced todate based on renewable raw materials, for example from celluloses, havesignificantly worse absorption and water-retention properties than thewater-absorbing polymers based on polyacrylates. This has, in turn, adisadvantageous effect on composites comprising these water-absorbingpolymers, in particular hygiene articles. These become as a rule morevoluminous, which leads to a larger waste volume and worsened wearercomfort, and, in addition, have worse water-retention properties andmore leakage.

Thus, a further object according to the invention consists in helping toalleviate the disadvantages described in the above paragraph or even toovercome them.

Furthermore, an object according to the invention consists in providingpolyacrylates and in particular water-absorbing polymers which aregentler on resources, which are not inferior in their physicalproperties to previous polyacrylates and in particular water-absorbingpolymers.

Furthermore, an object of the present invention is to providecomposites, in particular hygiene articles, which are acceptable from anecological point of view, which are not inferior in their properties toprevious composites and in particular hygiene articles.

A contribution to the solution of at least one of the above objects isprovided by the subject matters of the category-forming independentprincipal and adjacent claims, whereby the therefrom dependentsub-claims represent preferred embodiments of the present invention,whose subject matters likewise make a contribution to solving at leastone object.

SUMMARY

According to an embodiment, the invention relates to a process forproduction of acrolein, at least comprising the following steps:

-   -   (a) bringing an aqueous glycerine phase into an acrolein        reaction area to obtain an aqueous acrolein reaction phase;    -   (b) depleting the acrolein from the acrolein reaction phase to        obtain an acrolein phase and a depleted acrolein reaction phase;        and    -   (c) conducting back at least a part of the depleted acrolein        reaction phase into the acrolein reaction area.

According to another embodiment, the invention relates to a process forproduction of acrylic acid, comprising at least the following steps:

-   -   (A) bringing an aqueous glycerine phase into an acrolein        reaction area to obtain an aqueous acrolein reaction phase;    -   (B) depleting the acrolein from the acrolein reaction phase to        obtain an acrolein phase and a depleted acrolein reaction phase;    -   (C) conducting back at least a part of the depleted acrolein        reaction phase into the acrolein reaction area; and    -   (D) oxidation of the acrolein from the acrolein phase to acrylic        acid in the gas phase at a gas phase catalyst.

FIGURE

The foregoing and other features, aspects, and advantages of the presentinvention will become better understood with regard to the followingdescription, appended claims, and accompanying drawing where:

FIG. 1 shows schematically, a first embodiment of an arrangementaccording to the invention.

DETAILED DESCRIPTION

In general, in the conducting back, the back-conducted flow is adjustedso that high acrolein yields may be obtained with turnovers which are ashigh as possible. The return ratio of the glycerine phase to theconducted back depleted acrolein reaction phase may lie within the rangefrom about 0.01:10 to about 9:10, or from about 0.1:10 to about 5:10, orfrom about 0.5:10 to about 3:10. The conducting back serves to protectthe environment. For the case that no back-conducting occurs, thedepleted acrolein reaction phase must be removed in some other way. Thismay occur by dumping, in purification plants, or in combustion plants.Accordingly, the process according to the invention is also possiblewithout recycling, which may not be advantageous for environmentalreasons.

The acrolein reaction phase in the acrolein reaction area may have apressure of at least about 50, or at least about 80, or at least about120, or at least about 140 bar. The acrolein reaction area is thusdesigned as a pressure area, which is limited at its start by a pressuregenerator such as a pump and at its end by a pressure regulator, such asa pressure valve. The dehydration reaction may occur at least in a partof the acrolein reaction area. Generally, the acrolein reaction area maybe at least partially formed like a pipe, and designed for up to amaximum pressure load of about 500 bar and a maximum temperature load ofabout 600° C., which are sufficient for carrying out the processaccording to the invention.

In addition, the acrolein reaction phase in the acrolein reaction areamay have a temperature of at least about 80° C., or at least about 180°C., or at least about 230° C., or at least about 280° C., or at leastabout 320° C. The temperatures may, on the one hand, be achieved via thepressure ratios in the acrolein reaction area as well as via acorresponding heating of the acrolein reaction phase. In general, thepressure and/or temperature conditions in the acrolein reaction phase inthe acrolein reaction area may be selected so that the acrolein reactionphase and in particular the water comprised therein are at least closeto or at least partially in the supercritical region.

The glycerine phase may comprise less than about 10 wt %, or less thanabout 8 wt %, or less than about 6 wt % glycerine, based on the totalweight of the glycerine phase, whereby the minimum amount of glycerinein the glycerine phase is about 0.01 wt %, or about 0.1 wt. % or about 1wt. %.

In addition, the acrolein reaction area may comprise, besides water, adehydration catalyst. This may be present in an amount from about0.001:1000 to about 10:1000, or from about 0.01:1000 to about 5:1000 orfrom about 0.04:1000 to about 1:1000, respectively based upon the amountof glycerine used in the acrolein reaction phase.

The dehydration catalyst may be present either as an acid or as a baseor as a combination thereof. If the dehydration catalyst is present asacid, this acid may be a compound, besides water, which also acts as astrong acid close to or within the supercritical region, which hasacidic properties. If the dehydration catalyst is an acid, bothinorganic and organic acids may be considered. Inorganic acids mayinclude phosphorus acids such as H₃PO₄, sulphur acids such as H₂SO₄,boron acids such as B(OH)₃, or a mixture thereof. In a furtherembodiment of the dehydration catalyst, this is present as a superacid,which, according to the definition, has a small pK_(S) value of <−1. Ifthe dehydration catalyst is present as an organic acid, alkylsulfonicacids may be used, whereby trifluoromethanesulfonic acid ormethanesulfonic acid or mixtures thereof are examples. As bases areconsidered in connection with the dehydration catalyst, examples includealuminum, lanthanum, alkali or alkaline earth oxides, hydroxides,phosphates, pyrophosphates, hydrogen phosphates or carbonates, or amixture of at least two thereof, which may be respectively also be on acarrier.

Furthermore, the dehydration catalyst may be present at room temperatureboth as a solid as well as a liquid. Fluid dehydration catalystsimmobilized on a solid carrier also fall under the dehydration catalystspresent as solid. Solid dehydration catalysts may include siliconoxide-comprising compounds such as zeolites. In addition, Ti, Zr, or Ceoxides, sulfatized oxides, and phosphatised oxides, or mixtures of atleast two thereof are also considered.

A number of dehydration catalysts is described more closely in DE 42 38493.

The acrolein reaction phase may comprise a fluid different from water.For the case when fluid dehydration catalysts are used, this fluidshould also be different from these catalysts. These fluids have afunction as solubility improvers. In general, organic compounds that arewater-miscible at about 20° C. may be considered as such fluids, whichcomprise at least one hetero-atom, or two hetero-atoms, and may be inertwith respect to other components of the acrolein reaction phase. Suchfluids may include, for example, hydroxypiperidine, or aprotic, andpolar fluids such as sulfolane, diglyme, tetraglyme, dioxane, trioxane,or γ-butyrolactone. Furthermore, compounds are considered as fluids thathave a chelating effect.

In this context, EDTA, NTA, or DPTA are examples, as obtainable underthe trade names Versene®, Versenex®, Entarex®, or Detarex®, or alsocrown ethers.

In another embodiment, the acrolein reaction area may comprise a metal,or a metal compound, or both. This compound may be a mono-, di-, ormultivalent metal, or metal compounds. This metal or these metalcompounds may be different from the metal or metals which are used inthe construction of the acrolein reaction area. This also corresponds toan embodiment according to the invention that these metals or metalcompounds are immobilized directly or indirectly with assistance of anadhesive agent to the material used for the construction of the acroleinreaction area. These metals or metal compounds may, however, also bepresent in particulate form in the acrolein reaction area. These metalsor metal compounds should not be carried out of the acrolein reactionarea by a fluid or gas flow. This may be achieved, in addition to theimmobilization of these metals or metal compounds in the case where theyare present in particulate form, by suitable sieves or filters providedin the acrolein reaction area. Furthermore, it further corresponds to anembodiment of the process according to the invention that the metals ormetal compounds respectively may be selected so that the above-mentionedfluids may coordinate or complex to these metals or metal compounds. Inaddition, these metals may be present as metal compounds, whereby metalsalts or metals complexed with ligands are examples. As ligands areconsidered in particular carbon monoxide such as carbonyl,triphenylphosphine, Cp, Cp*, or AcAc are examples. The metal salts maybe used in particular in the form of their sulphates or phosphates.Metals may include tin, such as tin sulphate, zinc such as zincsulphate, lithium such as lithium sulphate, magnesium such as magnesiumsulphate, copper such as copper sulphate, palladium such as palladiumcarbonyl complex, which is mostly used as acetate, rhodium such asrhodium carbonyl complex, which is mostly used as acetate, rutheniumsuch as ruthenium carbonyl complex, which is mostly used as acetate,nickel such as nickel carbonyl complex, which is mostly used as acetate,iron such as iron carbonyl complex, cobalt such as cobalt carbonylcomplex, caesium such as caesium acetate as well as lanthanides,lanthanum, or a mixture of at least two thereof. The metals may be usedas salts with complexing agents, often also in the presence of carbonmonoxide. Heteropolyacids are examples of metal compounds. Examples ofheteropolyacids include those that arise if different types of acidicmolecules of a metal such as of chromium, tungsten, or molybdenum, and anon-metal, such as phosphorus, come together with discharge of water.Heteropolyacids may include for example, phosphorus-tungsten acids,silico-tungsten acids, or silico-molybdenum acids, and also thecorresponding vanadium compounds.

The dwell time of the acrolein reaction phase in the acrolein reactionarea may lie from about 1 to about 10,000 seconds, or from about 5 toabout 1,000 seconds, or from about 10 to about 500 seconds.

In addition, it has been shown to be helpful that the acrolein reactionphase comprises carbon monoxide from about 0.0001 to about 10 wt %, orfrom about 0.001 to about 7 wt %, or from about 0.005 to about 5 wt %,respectively based upon the acrolein reaction phase. This measure may beadvantageous for the reduction of side-components.

In addition, the acrolein reaction phase at the end of the acroleinreaction area may comprise an amount of less than about 50 wt %glycerine, or less than about 25 wt % glycerine, or less than about 15wt % glycerine, and an amount of from about 0.1 to about 50 wt. %, or offrom about 1 to about 40 wt %, or from about 5 to about 30 wt % ofacrolein, respectively based upon the acrolein reaction phase. By thisway of carrying out the process, an acrolein phase may be obtained thatmay be fed into step (D) over a substantially longer time, withoutnotable worsening of the conversion of acrolein to acrylic acid. It is,furthermore, generally the case in the process according to theinvention that the glycerine concentration at the start of the acroleinreaction area is greater than at the end of the acrolein reaction areaand may continuously reduce towards the end.

According to a particular embodiment of the process of the invention,the turnover in the acrolein reaction area is at least about 25%, or atleast about 26%, or at least about 30%, or at least about 50%. Aturnover of at least about 25% means here that at least about 25% of theglycerine molecules entering the acrolein reaction area are convertedinto acrolein.

At least a part of the acrolein reaction phase may be present in gaseousform. The acrolein reaction phase in the acrolein reaction area may bepresent in at least two aggregate states. These aggregate states may beliquid and gaseous. For the case that at least a part of the acroleinreaction phase is present as a gas, the concentration in acrolein inthis acrolein reaction gas phase may be higher than in the part of theacrolein reaction phase that has a different aggregate state to theacrolein reaction gas phase. A depletion or respective separation of theacrolein is possible considerably more simply by means of the highacrolein concentration in the acrolein reaction gas phase, in thatpredominantly the acrolein reaction phase from the acrolein reactionarea, which is highly concentrated in acrolein, may be discharged by acorresponding pressure regulation, and then acrolein may be obtained inhigh concentration by releasing pressure.

The purer the thus-obtained acrolein, the less it is necessary that, inaddition to the release of pressure, which may occur, for example, bymeans of a pressure regulator formed as a pressure regulating valve, acooling by means of a heat exchanger and a further separation, whichgenerally occurs distillatively, a separating unit is necessary. It isfurther possible that the acrolein reaction phase leaving the acroleinreaction area may be conducted via a plurality of units connected oneafter the other and consisting of an over-current valve and a heatexchanger, before the thus-created acrolein phase is conducted to aseparating unit. The pressure difference before the pressure regulatorin the acrolein reaction area, and after the pressure regulator, ispreferably at least about 30 bar, or at least about 60 bar, or at leastabout 100 bar. The acrolein in the acrolein reaction area may at leastpartially be present in a supercritical state, which contributes to theincreased yield.

The acrolein concentration in the acrolein reaction phase before thedepletion may be higher by at least about 5%, or at least about 10%, orat least about 50% than after the depletion. A carrier gas may be usedin the process. This carrier gas may be supplied before the acroleinreaction area and serves to discharge the acrolein reaction phase. Alsoin this context, it is advantageous to find as much acrolein as possiblein a gaseous part of the acrolein reaction phase. As carrier gas, inprincipal, all gases that are inert with respect to the compoundsparticipating in the above process may be considered. Examples forcarrier gases of this type include but are not limited to nitrogen, air,CO₂, water, or argon. The carrier gas may at least be partially fed backinto the acrolein reaction area after passing through the acroleinreaction area. This feed may occur directly before the acrolein reactionarea or also at any other position before the acrolein reaction area andmay be used in order to form a pre-pressure of the reactants, which arefurther compressed by means of a corresponding pump to the pressureconditions necessary for the acrolein reaction area.

In the process according to the invention for production of acrylicacid, the acrolein phase in step (D) may comprise acrolein of from about5 to about 30 wt %, or from about 7 to about 20 wt %, or from about 10to about 20 wt %, respectively based on the acrolein phase. Inconnection with as long a life as possible for the oxidation reactor instep (D), the acrolein phase may comprise less than about 10 wt %, orless than about 5 wt %, or less than about 2 wt % components which aregenerally described as high-boilers, and may have a higher boiling pointthan acrolein. The acrolein phase may comprise less than about 10 wt %,or less than about 5 wt %, or less than about 2 wt. %, respectivelybased on the acrolein phase, of low-boilers, i.e. materials which have alower boiling point than acrolein. In another embodiment, the acroleinphase, in addition to acrolein and optionally present low- orhigh-boilers, respectively, may comprise substantially inert components,in particular gaseous components, which only negatively affect theoxidation reaction according to step (D) insubstantially, if at all.

During the oxidation in step (D), an acrylic acid comprising gaseousacrylic acid phase arises, whereby acrylic acid is depleted from thisacrylic acid phase and at least a part of the depleted acrylic acidphase may be fed into step (A) or (D). Part of the depleted acrylic acidphase before the feeding-in may be subjected to a combustion, such as agas phase combustion and particularly preferably a catalytic gas phasecombustion, as described in WO 03/051809. A depleted acrylic acid phasepreferably comprises less than about 5 wt %, or less than about 1 wt %,or less than about 0.1wt % of acrylic acid, respectively based on thedepleted acrylic acid phase. Further components of the depleted acidphase may include water, nitrogen, and CO₂. Advantageously, the part ofthe depleted acrylic acid phase, in particular after the combustion, maybe used as carrier gas in the process according to the invention forproduction of acrylic acid. Furthermore, the oxygen or air flow,respectively necessary for an oxidation of the acrolein, may beintroduced either to be used at the same time as carrier gas in step (A)or for the purpose of the oxidation of the acrylic acid directly in step(D).

Furthermore, carbon monoxide may be supplied to the acrolein reactionphase, or if large amounts of carbon monoxide have been formed duringthe dehydration, that the carbon monoxide may be either selectivelyoxidized or removed before the bringing into contact with gas phasecatalyst, in order to prevent, in particular in the case of metal oxidesas gas phase catalyst, a reduction of the catalyst and thus an at leastpartial inactivation. The carbon monoxide may, for example, beselectively oxidized to carbon dioxide.

The invention further relates to an oxidation device, comprising,connected with each other in fluid-conducting manner,

-   -   a dehydration unit;    -   downstream therefrom, a gas phase oxidation unit;    -   whereby the dehydration unit comprises        -   a reactant feed;        -   downstream therefrom, an acrolein reaction area;        -   downstream therefrom, a pressure regulator; and        -   downstream therefrom, a depletion unit, whereby the            depletion unit is connected with the gas phase oxidation            unit in fluid-conducting manner;    -   whereby the gas phase oxidation unit comprises, downstream from        the depletion unit        -   a reactor, comprising a multioxide catalyst; and        -   a processing unit.

The reactant feed may occur by taking the reactant from a tank, whichmay receive either glycerine as such or glycerine in the form of anaqueous solution. In the context of the acrolein reaction area,reference is first made to the above details. The acrolein reactionarea, in the region in which it is formed like a pipe, may have a longerdiameter compared to the cross-section.

The pressure regulator following downstream from the acrolein reactionarea, from the viewpoint of the reactant feed and in the sense of theflow of reactants and reaction products, may have at least one, or atleast two or more pressure regulators, formed as pressure regulatingvalves—for example as an over-current valve. A depletion unit followsthis, in turn, downstream. The depletion unit may directly follow thepressure regulator. This may be used if the depletion of the acroleinfrom the acrolein reaction phase present before the pressure regulatoroccurs by release of pressure of the acrolein reaction phase. By thesemeasures, a further reaction of the acrolein phase may be reduced orcompletely prevented and thus also the formation of undesiredside-components.

According to another embodiment of the device according to theinvention, the depletion unit may comprise a heat exchanger. This may beprovided at the start of the depletion unit. In another embodiment ofthe device according to the invention, a separation device may followfrom the heat exchanger, which is formed as a membrane or crystallizerand in particular as a distillation column. The device according to theinvention, either in the acrolein reaction area or before the acroleinreaction area or at both positions, may include a heating element. Thisheating element may be thermally coupled with the heat exchangerprovided in the depletion unit.

The acrolein reaction area may further comprise a dehydration catalyst.This dehydration catalyst may be arranged and fixed in the acroleinreaction area. This may be achieved in that the dehydration catalyst isimmobilized at walls of the acrolein reaction area, or, if thedehydration catalyst is present in the form of particles or immobilizedthereon, suitable sieves and filters in the acrolein reaction areaprevent the flushing-out of these particles.

Furthermore, the oxidation device according to the invention in oneembodiment may comprise the multioxide catalyst as powder, layer, orpellet or a combination of at least two thereof. These powders, layers,or pellets may be located at metal walls of metal plates or metal pipes.In the device according to the invention, plate reactors, for examplethose with thermo plates, or with a plurality of pipes, also called pipebundle reactors, may be used. In connection with the composition of themultioxide catalysts, reference is made to the details in WO 03/051809as part of this disclosure, whereby catalysts based on molybdenum,vanadium, and tungsten may be used.

The processing unit may further comprise a quench unit. The deviceaccording to the invention may comprise a water separating unit, whichis preferably combined with the quench unit and contributesadvantageously to the generation of the acrylic acid-depleted acrylicphase, whereby in this context, references are also made to thedisclosure of WO 03/051 809.

In a further embodiment of the process according to the invention forproduction of acrylic acid, this occurs in an above-described device.

The invention also relates to a process for production of a polymer byradical polymerization of the acrylic acid comprising the steps:

-   -   i) provision of an optionally partially neutralized acrylic acid        and a monomer phase comprising cross-linker, whereby the acrylic        acid is obtained according to the above-described process;    -   ii) radical polymerization of the monomer phase to obtain a        hydrogel;    -   iii) optionally, comminution of the hydrogel;    -   iv) drying of the hydrogel to obtain a particulate        water-absorbing polymer structure;    -   v) optionally, milling of the particulate water-absorbing        polymer structure;    -   vi) surface post-cross linking of the particulate        water-absorbing polymer structure;    -   vii) bringing into contact of the water-absorbing polymer        structure with a coating agent, wherein the        bringing-into-contact occurs before, during, or after,        particularly preferably after the surface post-cross linking.

This radical polymerization may occur in the presence of cross linkersand using the acrylic acid in at least partially neutralized form, sothat in this way cross-linked, water-absorbing polymer structures may beobtained. With respect to the details of the production of suchwater-absorbing polymer structures based on acrylic acid, reference ismade to “Modern Superabsorbent Polymer Technology”, F. L. Buchholz andA. T. Graham, Wiley-VCH-Verlag. The acrylic acid in process step i) maybe present to at least about 20 mol %, or to at least about 50 mol %,based on the monomer, as a salt.

With respect to the preferred cross linkers and surface post-crosslinking agents, as well as with respect to the amounts and theconditions under which these components are used, as well as with regardto further components which may be present in a monomer solution, aswell as with regard to the polymerization conditions, the dryingconditions, the comminution, and the surface post-crosslinking,reference is made to DE 103 34 271 A1, whose disclosure limited to crosslinkers and surface post-cross linking agents consistent with thisinvention is hereby incorporated by reference.

As coating agent in process step vii), organic or inorganic materialsmay be used. As organic material, any optionally particulate organicmaterial known to the skilled person may be used, which is commonly usedfor modification of properties of water-absorbing polymers. Thoseorganic materials which are mentioned in DE 103 34 286 A1 as fineparticulate organic materials belong to the preferred organic materials.Besides these particulate organic materials, those compounds may also beused which are mentioned in WO 02/34384 A1 as nitrogen-containingnon-ionic surfactants, or also silicones, as described in EP 0 977 803A1.

As inorganic material, a particulate, inorganic material known to theskilled person may be used as coating agent, which is generally used tomodify the properties of water-absorbing polymers. Those inorganicmaterials which are mentioned in DE 103 34 286 A1 as fine particulateinorganic materials also belong to the preferred inorganic materialshere, whereby zeolites, silicon dioxides, and kaolin are particularlypreferred. Further preferred inorganic materials, preferably particulateinorganic materials, are phosphates, as mentioned in WO 02/060983 A2,and aluminum-comprising particles, which are mentioned, for example, inWO 2004/113452 A1, WO 2004/069293 A1, WO 2004/069915 A1, and WO2005/027986 A1.

The coating agents in process step vii) in an amount of from about 0.01to about 10 wt %, or in an amount from about 0.1 to about 5 wt %, basedon the weight of the water-absorbing polymer structures, may be broughtinto contact with these structures.

A contribution to the solution of the above-mentioned objects is alsomade by the water-absorbing polymer structures obtainable by the abovedescribed process.

A contribution to the solution of the above-mentioned objects is alsomade by water-absorbing polymer structures which are based to at leastabout 25 wt %, or to at least about 50 wt %, or to at least about 75 wt%, or to at least about 95 wt % on acrylic acid, whereby at least about80 wt %, or at least about 90 wt %, or at least about 95 wt % of theacrylic acid monomers used in the production of the water-absorbingpolymer structures have been obtained by the above-described processfrom glycerine via acrolein as intermediate product, and which have beencoated with about 0.01 to about 10 wt. %, based on the weight of thewater-absorbing polymer structures, of a coating agent, whereby examplesof coating agents are those coating agents that have already beenmentioned above in the context of the process according to the inventionfor the production of water-absorbing polymer structures.

The coating agent may not be a surface post-crosslinker.

According to a particular embodiment of the water-absorbing polymerstructures according to the invention, these are based to at least about25 wt %, or at least about 35 wt %, or at least about 45 wt % onnatural, biodegradable polymers, preferably on carbohydrates such as,for example, celluloses or starches.

It is further preferred according to the invention that thewater-absorbing polymer structures have at least one of the followingproperties:

-   (β1) a CRC value (CRC=Centrifugation Retention Capacity) determined    according to ERT 441.2-02 (ERT=Edana Recommended Test Method) of at    least about 20 g/g, or at least about 25 g/g, or at least about 30    g/g, whereby a CRC value of about 60 g/g, or of about 50 g/g is not    exceeded;-   (β2) an absorption under a pressure of 20 g/cm² determined according    to ERT 442.2-02 of at least about 16 g/g, or at least about 18 g/g,    or at least about 20 g/g, whereby a value of about 50 g/g, or about    40 g/g is not exceeded;-   (β3) the polymer structure has a biodegradability determined    according to the modified Sturm test according to Appendix V to the    Guideline 67/548/EWG after 28 days of at least about 25%, or at    least about 35%, or at least about 45%, whereby a value of at most    about 75 to about 95% as upper limit is generally not exceeded.

A further contribution to the solution of the above-described objects isprovided by a composite comprising the water-absorbing polymerstructures according to the invention or respectively water-absorbingpolymer structures which may be obtainable by radical polymerization ofthe acrylic acid obtainable by the above-described process in thepresence of crosslinkers. The polymer structures according to theinvention and the substrate may be firmly bound to each other. Assubstrate, sheets made from polymers, such as, for example, frompolyethylene, polypropylene or polyamide, metals, non-wovens, fluff,tissues, woven materials, natural, or synthetic fibers, or other foamsmay be used. The polymer structures may be comprised in an amount of atleast about 50 wt %, or at least about 70 wt %, or at least about 90 wt%, based on the total weight of polymer structures and substrate, in thecomposite.

In a particularly preferred embodiment of the composite according to theinvention, it is a sheet-like composite, as described in WO-A-02/056812as absorbent material. The disclosure of WO-A-02/056812, in particularwith respect and limited to the exact construction of the composite, themass per unit area of its components and its thickness is herebyintroduced as reference and represents a part of the disclosure of thepresent invention.

A further contribution to the solution of the above-mentioned objects ismade by a process for production of a composite, whereby thewater-absorbing polymer structures according to the invention orrespectively the water-absorbing polymers which may be obtainable byradical polymerization of the acrylic acid obtainable by theabove-described process in the presence of cross-linkers, and asubstrate, and optionally an additive may be brought into contact witheach other. As substrate, those substrates may be used that have alreadybeen mentioned in connection with the composite according to theinvention.

A contribution to the solution of the above-mentioned object may also bemade by a composite obtainable according to the above-described process.

A further contribution to the solution of the above-mentioned objectsmay be made by chemical products comprising the water-absorbing polymerstructures according to the invention or a composite according to theinvention, or based on the acrylic acid obtainable by the processaccording to the invention. Examples of chemical products may includefibers, sheets, formed masses, textile and leather additives,flocculants, coatings, varnishes, foams, films, cables, sealantmaterials, liquid-absorbing hygiene articles, in particular diapers andsanitary napkins, carriers for plant or fungus growth-regulating agentsor plant protection active agents, additives for construction material,packaging materials, or soil additives.

Hygiene articles according to the invention may comprise a top sheet, abottom sheet, and an intermediate sheet arranged between the top sheetand the bottom sheet, which may comprise the water-absorbing polymerstructures according to the invention.

The use of the water-absorbing polymer structures according to theinvention or of the composite according to the invention in chemicalproducts, in the above-mentioned chemical products, in particular inhygiene articles such as diapers or sanitary napkins, as well as the useof the water-absorbing polymer structures as carrier for plant or fungusgrowth-regulating agents or plant protection active materials also makea contribution to the solution of the above-mentioned objects. In theuse as carrier for plant or fungus growth-regulating agents or plantprotection active substances, it is preferred that the plant or fungusgrowth-regulating agents or plant protection active substances may bereleased over a time period controlled by the carrier.

The present invention is now more closely described by means ofnon-limiting diagrams and examples.

FIG. 1 shows schematically a device 1 according to the invention fordehydration and oxidation, comprising a dehydration unit 2, which isconnected with a gas phase oxidation unit 3 in fluid-conducting fashion,i.e. connected flow-technologically with each other in such a way thatboth liquid and gas may be conducted. The dehydration unit 2 receives,via a reactant feed 4, glycerine or respectively an aqueous solution ofglycerine, which may be pre-stored in a tank which is not shown. Bymeans of a pressure generator 23 designed as a high pressure pump (forexample a multipiston pump from the company Lewa, Germany) the aqueousglycerine in an acrolein reaction area 5 (such as a stainless steelpipe) is compressed against a pressure regulator 6 (for example formedas over-current valve) and, if necessary, further heated by means of aheating element 12. The acrolein reaction area 5 may further comprise adehydration catalyst 13 immobilized therein, or liquid catalyst may besupplied, at which the glycerine reacts to form acrolein. By means ofthe pressure regulator 6, the thus-formed acrolein is discharged fromthe acrolein reaction area 5 which is under high pressure by release ofpressure into a depletion unit 7. The depletion unit 7 may in turncomprise a heat exchanger 11, which is thermally coupled with theheating element 12. In the depletion unit 7, a distillation device 24may follow from the heat exchanger 11 usable for the cooling. Anacrolein-poor acrolein reaction phase leaves the depletion area 7 and inparticular the distillation device 24 via a back-conduit 21, in order tobe supplied via reactant feed 4 to the acrolein reaction area 5, inorder to conduct the glycerine still present in the acrolein-pooracrolein reaction phase to a further dehydration. Furthermore, anacrolein-rich acrolein phase leaves the depletion unit 7 into the gasphase oxidation unit 3 following the depletion unit 7. The gas phaseoxidation unit 3 comprises, in turn, a reactor 9, which comprises, inpipe walls represented schematically as pipe cross-section, catalystpowder 14 or a catalyst layer 15 or catalyst pellets 16. A processingunit 10 follows the reactor 9. This processing unit comprises a quenchunit 17 formed as a quench column and a water separating unit 18. Fromthe processing unit 10, via a back-line 20 or 20′ respectively, anacrylic acid-poor acrylic acid phase may be supplied to the reactantfeed 4 or respectively to the reactor 9. An acrylic acid-rich acrylicacid phase is supplied from the processing unit 10 to a purificationunit 19, which is, for example, designed as crystallization unit, asdescribed in DE 102 11 686. The acrylic acid obtained here from in highpurity may, furthermore, be further processed to polyacrylates and inparticular also as water-absorbing polymers characterized assuperabsorbers.

EXAMPLE 1

A glycerine solution (5 wt. % in water, acidified with phosphoric acidin the ratio 1:2000, based on the glycerine) was supplied at 360 ml/hinto a reactor (acrolein reaction area 5) with a volume of 95 ml. Thepressure in the reactor was maintained at 150 bar. The reactor wasbrought to a temperature with a maximum of 345° C. by means of secondaryheating. The turnover in first throughput was 89.6%, the selectivity foracrolein was 80.2%, and the yield of acrolein in the first throughputwas 71.8%. The phase from which acrolein was removed was conducted backinto the reactor for simulation of a continuous circuit.

EXAMPLE 2

A glycerine solution (5 wt. % in water, acidified with phosphoric acidin the ratio 1:2000, based on the glycerine) was fed at 480 ml/h into areactor with a volume of 95 ml. The pressure in the reactor wasmaintained at 150 bar. The reactor was brought to a temperature with amaximum of 345° C. by means of secondary heating. The turnover was29.5%, and the selectivity for acrolein was 73.7%.

EXAMPLE 3

The hot product stream at 180-220° C. in the form of vapor from thedehydration reactor, with a composition of 15 wt. % acrolein, 82 wt. %water vapor, and the remainder other lower boiling components was,analogously to WO 03/051809 A1, together with 1.5 kg/h pre-heated air,fed into an oxidation reactor which is filled with 1.8 1 commercial V-Momultioxide catalyst.

The acrolein/water vapor/air mixture from the dehydration reactor wasconverted at 250° C. and slightly increased ambient pressure with a GHSVof 280 Nl acrolein/(1 cat·h) and in the reactant mixture, with anacrolein turnover of 99.5 mol %, an acrylic acid yield of 93 mol % wasobtained.

EXAMPLE 4

A monomer solution consisting of 280 g of the above obtained acrylicacid, which was neutralized to 70 mol % with sodium hydroxide, 466.8 gwater, 1.4 g polyethylene glycol-300-diacrylate, and 1.68 gallyloxypolyethylene glycol acrylic acid ester was purged with nitrogento remove dissolved oxygen and cooled to a starting temperature of 4° C.After reaching the starting temperature, the initiator solution (0.1 g2,2′-azobis-2-amidinopropane dihydrochloride in 10 g H₂O, 0.3 g sodiumperoxydisulfate in 10 g H₂O, 0.07 g 30% hydrogen peroxide solution in 1g H₂O, and 0.015 g ascorbic acid in 2 g H₂O) was added. After the endtemperature of approximately 100° C. was reached, the resulting gel wascomminuted and dried for 90 minutes at 150° C. The dried polymer wascoarsely chopped, milled, and sieved to a powder with a particle sizefrom 150 to 850 μm.

For the cross-linking, 100 g of the above-obtained powder was mixed withvigorous stirring with a solution of 1 g 1,3-dioxolan-2-one, 3 g waterand 0.5 g aluminum sulphate-18-hydrate, and then heated for 40 minutesin an oven which was regulated to 180° C.

After cooling, the water-absorbing polymer particles are sprayed with a50% aqueous slurry of Kaolin (NeoGen, DGH®) in such an amount that thewater-absorbing polymer structure was coated with 3 wt. % Kaolin.

EXAMPLE 5 Preparation of a Biodegradable Polymer

The post-crosslinked polymer surface-treated with kaolin obtained inExample 4 was mixed under dry conditions with a water-soluble wheatstarch (the product Foralys® from the company Roquette, Lestrem, France)in the weight ratio polymer:starch of 4:1 and then further homogenizedfor 45 minutes in a roll mixer type BTR 10 from the company Fröbel GmbH,Germany.

LIST OF REFERENCE NUMERALS

-   1 oxidation device-   2 dehydration unit-   3 gas phase oxidation unit-   4 reactant feed-   5 acrolein reaction area-   6 pressure regulator-   7 depletion unit-   8 reactor-   9 multioxide catalyst-   10 processing unit-   11 heat exchanger-   12 heating element-   13 dehydration catalyst-   14 powder-   15 layer-   16 pellet-   17 quench unit-   18 water separation unit-   19 purification unit-   20, 20′ conducting back of the acrylic acid-poor acrylic acid phase-   21 conducting back of the acrolein-poor acrolein reaction phase-   22 CO-feed-   23 pressure generator-   24 distillation device

1. A process for production of acrolein comprising the following steps:(a) bringing an aqueous glycerine phase into an acrolein reaction areato obtain an aqueous acrolein reaction phase wherein the acroleinreaction phase is at least partially in the supercritical area; (b)depleting the acrolein from the acrolein reaction phase to obtain anacrolein phase and a depleted acrolein reaction phase; and (c)conducting back at least a part of the depleted acrolein reaction phaseinto the acrolein reaction area.
 2. A process for production of acroleinhaving the following steps: (a) bringing an aqueous glycerine phase intoan acrolein reaction area to obtain an aqueous acrolein reaction phase,wherein the acrolein reaction phase in the acrolein reaction area has apressure of at least about 80 bar and a temperature of at least about320° C.; (b) depleting the acrolein from the acrolein reaction phase toobtain an acrolein phase and a depleted acrolein reaction phase; and (c)conducting back at least a part of the depleted acrolein reaction phaseinto the acrolein reaction area.
 3. A process for production of acrylicacid, comprising the following steps: (A) bringing an aqueous glycerinephase into an acrolein reaction area to obtain an aqueous acroleinreaction phase; (B) depleting the acrolein from the acrolein reactionphase to obtain an acrolein phase and a depleted acrolein reactionphase; (C) conducting back at least a part of the depleted acroleinreaction phase into the acrolein reaction area; and (D) oxidizing theacrolein from the acrolein phase to acrylic acid in the gas phase at agas phase catalyst.
 4. The process according to claim 3, wherein theacrolein reaction phase in the acrolein reaction area has a pressure ofat least about 50 bar.
 5. The process according to claim 3, wherein theacrolein reaction phase in the acrolein reaction area has a temperatureof at least about 100° C.
 6. The process according to claim 3, whereinthe acrolein reaction area comprises a dehydration catalyst.
 7. Theprocess according to claim 6, wherein the dehydration catalyst is anacid or a base.
 8. The process according to claim 7, wherein the acid isan inorganic acid.
 9. The process according to claim 7, wherein the acidis an organic acid.
 10. The process according to claim 3, wherein theacrolein reaction phase comprises a liquid different from water.
 11. Theprocess according to claim 10, wherein the liquid different from wateris aprotic and polar.
 12. The process according to claim 3, wherein theacrolein reaction area comprises a metal or a metal compound or both.13. The process according to claim 3, wherein the residence time of theacrolein reaction phase is from about 1 to about 10000 seconds.
 14. Theprocess according to claim 3, wherein the acrolein phase comprisescarbon monoxide.
 15. The process according to claim 3, wherein theglycerine phase comprises less than about 10 wt % glycerine.
 16. Theprocess according to claim 3, wherein the turnover in the acroleinreaction phase is at least about 25%.
 17. The process according to claim3, wherein the acrolein reaction phase at the end of the acroleinreaction area comprises an amount of less than about 50 wt. % glycerin,based on the acrolein reaction phase.
 18. The process according to claim3, wherein the acrolein reaction phase at the end of the acroleinreaction area comprises an amount within the range from about 0.1 toabout 50 wt. % of acrolein, based on the acrolein reaction phase. 19.The process according to claim 3, wherein at least a part of theacrolein reaction phase is gaseous.
 20. The process according to claim3, wherein the acrolein reaction phase in the acrolein reaction area ispresent in at least two aggregate states.
 21. The process according toclaim 3, wherein the acrolein reaction phase before the depletion isunder higher pressure than during the depletion.
 22. The processaccording to claim 3, wherein the acrolein in the acrolein reaction areais at least partially present in the supercritical state.
 23. Theprocess according to claim 3, wherein the acrolein concentration in theacrolein reaction phase before the depletion is at least about 5% higherthan after the depletion.
 24. The process according to claim 3, whereina carrier gas is used.
 25. The process according to claim 24, whereinthe carrier gas is at least partially fed back into the acroleinreaction area after passing through the acrolein reaction area.
 26. Theprocess according to claim 3, wherein the acrolein phase in step (D)comprises acrolein within a range from about 5 to about 30 wt. %, basedon the acrolein phase.
 27. The process according to claim 3, whereinduring the oxidation an acrylic acid-comprising gaseous acrylic acidphase forms, wherein acrylic acid is depleted from this acrylic acidphase and at least a part of the depleted acrylic acid phase is fed intostep (A), or (D), or both.
 28. A device for dehydration and oxidation,connected with each other in fluid-conducting manner, comprising adehydration unit; downstream therefrom a gas phase oxidation unit;wherein the dehydration unit comprises a reactant feed; downstreamtherefrom an acrolein reaction area; downstream therefrom a pressureregulator; and downstream therefrom a depletion unit, wherein thedepletion unit is connected in fluid-connecting manner with the gasphase oxidation unit; wherein the gas phase oxidation unit comprises,downstream from the depletion unit a reactor, comprising a multioxidecatalyst; and a processing unit.
 29. The device according to claim 28,wherein the depletion unit comprises a heat exchange.
 30. The deviceaccording to claim 28, wherein the acrolein reaction area can be heatedby means of a heating element.
 31. The device according to claim 28,wherein the acrolein reaction area comprises a dehydration catalyst. 32.The device according to claim 28 wherein the dehydration catalyst isimmobilized in the acrolein reaction area,
 33. The device according toclaim 28, wherein the multioxide catalyst is present as powder, layer orpellet or a combination of at least two therefrom.
 34. The deviceaccording to claim 28, wherein the processing unit comprises a quenchunit.
 35. The device according to claim 28, wherein the processing unitcomprises a water separation unit.
 36. (canceled)
 37. A process forproduction of water-absorbing polymer structures, comprising the processsteps: i. provision of an optionally partially neutralized acrylic acidand a monomer phase comprising crosslinker, wherein the acrylic acid isobtained according to a process according to claim 3; ii. radicalpolymerization of the monomer phase to obtain a hydrogel; iii.optionally, comminution of the hydrogel; iv. drying the hydrogel toobtain a particulate water-absorbing polymer structure; v. optionally,milling of the particulate water-absorbing polymer structure; vi.surface post-crosslinking of the particulate water-absorbing polymerstructure; and vii. bringing into contact of the water-absorbing polymerstructure with a coating agent, wherein the bringing into contact occursbefore, during or after the surface post-crosslinking.
 38. The processaccording to claim 37, wherein the acrylic acid is present to at leastabout 20 mol % based on the monomer, as a salt.
 39. Water-absorbingpolymer structures, obtainable by a process according to claim
 37. 40. Awater-absorbing polymer structure, which is based to at least about 25wt. % on acrylic acid, wherein at least about 80 wt. % of the acrylicacid monomer used in the production of the water-absorbing polymerstructures, has been obtained by the process according to claim 3, andwhich is coated with from about 0.01 to about 10 wt. %, based on theweight of the water-absorbing polymer structures.
 41. Thewater-absorbing polymer structure according to claim 40, wherein thepolymer structure is based to at least about 25 wt. %, based on thetotal weight of the water-absorbing polymer structures, on natural,biodegradable polymers.
 42. A composite including a water-absorbingpolymer structure according to claim 39 and a substrate.
 43. A processfor production of a composite according to claim 42, wherein thewater-absorbing polymer structure and the substrate are brought intocontact with each other.
 44. A composite obtainable by a processaccording to claim
 43. 45. A hygiene article, comprising a top sheet, abottom sheet and an intermediate sheet, arranged between the top sheetand the bottom sheet, which includes water-absorbing polymer structuresaccording to claim
 39. 46. Fibers, sheets, formed masses, textile andleather additives, flocculants, coatings, or varnishes based on acrylicacid obtainable according to a process according to claim 3 or 36 orderivatives, or salts thereof.
 47. Use of an acrylic acid obtainableaccording to a process according to claim 3 or derivatives, or saltsthereof in fibers, sheets, formed masses, textile, and leatheradditives, flocculants, coatings, or varnishes.